The present invention relates to a cutting insert for chip forming machining, in particular milling cutters. The insert is produced by form pressing and sintering of an insert-forming powder. The insert comprises an upper chip surface, a lower, planar bottom surface which can be placed into abutment with a cooperating bottom support surface of the machining tool, and at least three side surfaces extending between the upper and bottom surfaces. At least one of the side surfaces adjoins the chip surface along a line that forms a cutting edge.
Such cutting inserts are more and more made by a direct-pressing method during which a cemented carbide forming powder first is shaped into desired form in a suitable pressing tool and then is given final strength by sintering in an oven at temperatures above 1000.degree. C. The pressing operation has been sophisticated over the years and is nowadays so well defined that the process provides good possibilities for forming the cutting edge and adjacent chip forming surfaces and possible reinforcing chamfers and clearance surfaces with great exactitude. Moreover, shrinkage occurring during the process is taken into account during the dimensioning of the pressing tool.
Today's cutting geometries tend to be more and more positive, i.e., larger and larger angles between the chip or rake surface of the insert and the normal to the machined surface. The reason for this geometry is that several advantages are achieved, such as a small cutting force and thus a low energy consumption, a well defined cutting edge for high measure precision, and a wider liberty when selecting the relief angle while maintaining a positive cutting geometry. In practice, the limit for the positivity of the angle of the chip surface is determined by the strength of the cemented carbide, since the edge angle becomes sharper, and thus weaker, the more positive the angle of the chip surface is.
In some circumstances, a negative axial inclination of the cutting insert in the milling cutter body can be advantageous, although the effective rake angle against the work piece is kept positive. This arrangement is often the case at the machining of cast iron, in particular in the motor industry, but the arrangement may also be advantageous for steel machining. Negative axial inclination angles are mainly used in end mills with small diameters. The negative inclination angle makes it possible to attain cutting inserts with up to eight cutting edges. The magnitude of the inclination can be up to 15.degree..
Also the clearance plays a decisive role at all cutting machining operations. In order to guarantee a sufficient free space in relation to the work piece, it is not permissible that too large a part of the clearance of relief surface behind the cutting edge in the cutting direction be in contact with the work piece. Insufficient clearance quickly results in a higher degree of face wearing and vibrations, and also in chipping and breakage of the cutting edge.
A negative inclination entails that the clearance of a plane relief surface increases with increasing cutting depth. Thereby, vibration problems occur at sharp (up to 90.degree.) new cutting edges before these have been "run in", i.e., before they have been submitted to a certain initial wear on the outermost cutting edge. Therefore, it is common practice for the machine operator to hone the new cutting edges to thereby increase the support surface and lower the vibration tendency at start up.
A first object of the present invention is to form a relief surface of a cuffing insert in such a way that the clearance can be adjusted to an optimal level at different cutting depths at negative axial inclination of the cutting insert in the milling cutter body.
A second object of the present invention is to maintain a substantially equal clearance along the cutting edge at negative axial inclination of the cutting insert in the milling cutter body.
A further object of the present invention is diminish, or even eliminate, the initial vibration tendencies at start up with completely new cutting edges.
These and other objects have been achieved by the present invention by providing a relief surface between two cutting comers which is helically twisted in such a way that the clearance (or relief angle) decreases with increasing cutting depth.
Indexable cutting inserts with twisted relief surfaces are previously known per se, see for instance EP-A-239 045 and EP-A-392 730. However, the relief surfaces of these inserts are twisted in a direction opposite to that of the present invention, i.e., the twist provides that the clearance increases with increasing cutting depth. Therefore, these prior art arrangements are not at all suitable for providing an insert that can be arranged inclined axially negatively in the milling cutter body. Moreover, these known cutting inserts also have a helically twisted chip surface in order to form a basically constant edge angle along the whole main cutting edge. However, that form results in the inconvenience of reducing the edge angle, which in turn can cause chipping and breakage of the cutting edge.